This is a summary of what I found in the rather longwinded topic
I would welcome any corrections and questions.
It was prepared offline with markdown
Some things I have learnt about s6 init system with s6-rc service manager
This is not a tutorial.
I am a long way from mastering s6-rc, and I have only progressed as far as I
have demonstrated here with generous assistance from @ProwlerGr
S6 on its own is an init system, and a supervisor system for services. That is it starts services when you boot, including the initial s6-svscan process with PID=1 which replaces the traditional ‘init’ process. S6 also supervises all running daemons, in the same manner as runit does, by running supervisor daemons for each service daemon. What supervision does is make sure the daemons stay running, by restarting them if they fail.
On top of S6, one can install a Service Manager. A Service Manager adds a facility for a user to start and stop services ( and a few other things). For example one might wish to temporarily stop the sshd daemon for security reasons.
I have seen 3 different Service Managers used with s6 : s6-rc, s6-66, and OpenRC. They all work. We have been looking at s6-rc in this topic.
s6-rc features
s6-rc is the original Service Manager supplied by Skarnet to work with s6.
I have seen s6-rc in Artix, but its best implementation is probably in the
init-diversity spin releases (Antix, MX, Devuan) which are all Debian based.
There are two concepts in s6-rc that are not prominent in other service managers
- compiling a database: all service managers have a database, but s6-rc compels the user to manage the database by hand. Other service managers hide the database.
- bundles: s6-rc has the concept of bundling services together so that they can be managed with a single command. I know of no other init system that has bundles. s6-66 has a related concept called a tree. SysVinit has runlevels which are difficult to configure and are generally unused.
Apart from these two major concepts, s6-rc has all the normal start , stop and status commands for daemons, plus a way of recompiling its database, which must be done after any changes.
s6-rc scripts
Most init systems have run scripts. These are usually shell scripts, but in s6-rc they are execline scripts
What a run script does is start a daemon to provide a service.
S6-rc keeps its ru scripts in /etc/s6-rc/sv. There you will find a directory for each service which has been installed, and inside that directory will be a run script plus other things. For example
$ls /etc/s6-rc/sv/sshd-srv
producer-for run type
and the run file contains
$ cat /etc/s6-rc/sv/sshd-srv/run
#!/bin/execlineb -P
##if -t { test -s /etc/ssh/sshd_not_to_be_run } exit 161
foreground { mkdir -p /run/sshd }
foreground { chmod 0755 /run/sshd }
foreground { redirfd -w 1 /dev/null ssh-keygen -A }
foreground { touch /var/log/lastlog }
foreground { chgrp utmp /var/log/lastlog }
foreground { chmod 664 /var/log/lastlog }
envfile /etc/s6-rc/config/openssh.conf
importas -sCuD "" OPTS OPTS
fdmove -c 2 1
exec /usr/sbin/sshd -D -e ${OPTS}
That is an execline script. Execline is like the primitive Unix rc command - it looks at a file and treats each line as a command and runs it. Only execline has some of its own builtin commands, so it is more sophisticaed than rc.
Try to avoid having to write an execline script from scratch. Most of the commonly used services have a run script and other things supplied when their package is installed. If your service does not have a script, try to find or modify one. See my difficulties here
with the vsftpd service.
s6-rc commands
There are only a few important commands
- s6-rc for managing services
https://skarnet.org/software/s6-rc/s6-rc.html
- s6-rc-db for managing the database
https://skarnet.org/software/s6-rc/s6-rc-db.html
- s6-svstat
https://skarnet.org/software/s6/s6-svstat.html
- the script s6-db-reload which is used to remake the database if you
change any of the files in `/etc/s6-rc/sv/…
There are no man pages, there is only the skarnet webpages referenced above plus a rather limitred beginners guide
and there are wiki pages from Artix and Gentoo
https://wiki.artixlinux.org/Main/S6
https://wiki.gentoo.org/wiki/S6_and_s6-rc-based_init_system
s6-rc Service Manager GUI
All 3 init-diversity spins come with a GUI service manager for s6-rc ( and there is one for runit and for sysVinit).
The s6-rc GUI can start and stop individual daemons. You have to add each daemon to the GUI bundle ( called antix-service-manager) first. You can use the GUI to do the add operation.
In addition the s6-rc GUI displays the status ( up or down) of each daemon.
The GUI automatically recompiles the database for you. If you use the CLI you have to do it by hand. That is what the script s6-db-reload is for.
the bundles concept in s6-rc
Individual daemons can be grouped together. For example the server daemon and the log daemon for a service an be bundled and managed with one command.
In my testing I managed to bundle vsftpd-srv and vsftpd-log together in to a bundle called vsftpd.
Commands work on bundles, the same way they work on individual services , so I was able to say
s6-rc start vsftpd
and start both daemons at once.
This takes some configuring in /etc/s6-rc/sv and it can not be done with the GUI at present.
s6-rc for a beginner
You will have no trouble with s6 itself. The init system and supervision are not requiring user intervention, they just work ( like in runit).
If you use s6-rc, there will be no difficulties if you stick to services for which the package system provides full s6-rc support.
If you go beyond that , be prepared for a lengthy learning curve. I have been assured that s6-66 is both simpler and superior, compared to s6-rc, so you should probably use s6-66 instead of s6-rc.